1. Field of the Invention
The present invention relates to a system for controlling a serial bus in a computer network so that any one of plural stations connected to the serial bus can be occupied.
2. Description of the Prior Art
Heretofore, as serial bus controlling systems of this type, there have been known a token pass system and a CSMA system. According to the former, the right of occupancy (called token) is allowed to cycle to stations, i.e., host computers, in good order. According to the latter system, each station can request the right of occupancy of a serial bus freely if necessary. FIG. 1(a) shows a conventional token pass system, in which T1, T2 and T3 each represent a station. And in FIG. 1(b), T.sub.t1, T.sub. t2 and T.sub.t3 indicate a state transition (time zone) of the right of occupancy for a bus TB. The stations T1, T2 and T3 can each occupy the bus TB in good order along the timing of the host computers T.sub.t1, T.sub.t2 and T.sub.t3 which have a state transition of the right of occupancy.
In other words, in the absence of any time zone of the state transitions T.sub.t1, T.sub.t2 and T.sub.t3 of the right of occupancy, the stations T1, T2 and T3 cannot occupy the bus even when the bus is in a state desiring data.
In the serial bus control according to the above token pass system, each station can occupy the bus only at the time zone allocated to itself; in other words, except this time zone, it cannot occupy the bus even in case of emergency. According to the CSMA system, it is possible to request the occupancy of bus if necessary, but it is not sure whether the right of bus occupancy will be obtained or not.